cells: Fix message counting in case of queue overflow

In case of message queue overflow, the message counters were not
decremented as they should.

Target: trunk
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6992/
(cherry picked from commit 360a2302a77cde3eac52e5d30f3fd9ae33b98872)
Merge pull request #558 from gbehrmann/fix/2.9/rb6992

Fix/2.9/rb6992